1. Field of Invention
This invention relates to transceivers, and more particularly to identical transceivers adapted for one in a slave mode to lock onto a frequency offset from the frequency of a related transceiver in a master mode by their common IF frequency.
2. Description of the Prior Art
A recent innovation in communications has been the utilization of microwave transceivers for line-of-sight transmission, typically as an alternative to hardwired connections between transmitting and receiving units. The apparatus may be utilized at extremely high frequencies, with carriers in the millimeter wave bands, thus providing them with a rather directional transmission characteristic which renders them useful in providing relatively secure transmission as well as avoiding interference with adjacent units in crowded areas (such as in building-to-building installations in cities).
In order to maximize utilization of such units, the cost thereof has been minimized in some cases by employing single oscillator designs in which a portion of the transmitter oscillator energy is coupled to the receiver as a substitute for a local oscillator to provide the beat frequency for generating the IF. In my aforementioned basic application, the use of a single solid state oscillator necessitates cancellation of the transmitter input modulation from the receiver output, particularly in the case of transmission and reception of data where any amount of crosstalk is intolerable. This is accomplished therein by controlling the gain of the transmitter input modulation amplifier in such a fashion as to null the transmitter input modulation from the receiver output on a closed-loop, AGC basis. However, the gains in the circuit are not properly adjusted until some time after transmitter input modulation is received.
Furthermore, depending upon the characteristics of the solid state oscillator in use, it may have insufficient open loop stability to meet FCC carrier frequency stability requirements. Such devices also frequently have an extremely wide tuning range. The frequency of oscillation of the voltage tunable solid state oscillator may readily be stabilized by means of a feedback loop including a resonant device together with means for initially sweeping the tuning voltage until the oscillator can lock onto the resonant frequency. In my aforementioned similar application, the frequency is stabilized by means of a resonant cavity feeding a synchronous demodulator, the transmitter input modulation being applied to the oscillator appears as sidebands in the wave passed through the cavity, which sidebands provide polarity or sense to the correction voltage such that it nulls the oscillator to the desired frequency as indicated by the cavity. However, in this case also, the stabilizing of oscillator operation at a desired frequency with respect to the resonant cavity cannot be established until transmitter input modulation is applied to the transceiver. In the case of covert operation, it is of course desirable to have both the frequency of oscillation and the closed loop AGC gain (for cancellation of unwanted receiver modulation) fully stabilized before it is desired to transmit.
In the case of matched transceivers which are designed for duplex operation, there are additional problems with the slave transceiver, which is typically locked (by AFC) to a frequency separated from the master transceiver frequency by the IF frequency of both transceivers, such that the slave receiver operates on the upper sideband of the master transmitter frequency while the master receiver operates on the lower sideband of the slave transmitter (or vice versa). The slave transceiver must have its oscillator swept in frequency until it can lock onto an AFC signal generated in its receivers, as a result of reception of a signal having a frequency separated from the master transmitter by the IF frequency. However, the sweeping of the input voltage which controls the frequency of a slave oscillator can cause it to lock onto other transceivers operating at extremely divergent frequencies, rather than to the master transceiver with which it is designed to operate as a pair. Even with limited voltage ranges custom designed for individual oscillators, the slave may lock onto the opposite sideband of a transceiver operating at a frequency separated by substantially twice the IF frequency of the pair. As a distinct problem, any receiver can respond to the joint transmissions of any two other transmitters operating in the same area and separated in frequency by an amount nearly the same as the IF frequency of the receiver, even if it doesn't lock on to either of them.